1. Field of the Invention
This invention relates to in-situ remediation of soils contaminated with volatile organic compounds. In particular, it relates to an improvement in an apparatus and method for vapor extraction of volatile organic compounds in soils.
2. Description of Related Art
Petroleum compounds, petroleum fractions and other organics have contaminated soils at thousands of sites due to spillage from above-ground sources or leakage from underground storage tanks. The spilled and leaked contaminants are adsorbed onto soil particles. Under the action of gravity these contaminants can reach great depths. If no efforts are made to remove these organic contaminants, they can migrate to aquifers where they threaten the purity of domestic, agricultural and industrial water supplies. The protection of groundwater sources and public health has become a major driving force in the enactment of laws and regulations to remediate such sites.
Applicable remediation systems include in-situ and ex-situ biological, chemical, thermal and physical systems. The high cost of excavating and transporting contaminated soils to an ex-situ treatment facility has focused attention on in-situ remediation systems. When the contaminating organic compounds have volatile components, a preferred physical in-situ remediation system is the soil vapor extraction system for reduction-or elimination of such contamination.
This system employs an air extraction well extending from the ground surface down to the contaminated soil zone, in conjunction with several air influx wells to a similar depth preferably located immediately outside the contaminated zone. An air displacement apparatus, such as an air extraction pump, pulls on the air extraction well and causes an air circulation downward in the influx wells, laterally through the contaminated soil zone, and upward in the extraction well.
To achieve the air circulation described above, each air influx well, typically a vertical cylindrical duct, has an opaque upper section down to the contamination zone, with a lower perforated section at the depth of this zone. The air flows down each influx well and through the perforations located near its bottom through the contaminated soil zone to the extraction well inlet. The extracted air carries with it volatile organic compounds stripped from the contaminated soil zone by the air circulation, and is usually directed to a utilization unit (such as an internal combustion engine) or an immobilization unit (such as an activated carbon bed) for contaminant disposal.
Usually, the cross-sectional flow area of the perforations is the same as the cross-section of the opaque upper section of the air influx well. A search in the recent (1991-1994) patent literature disclosed several vapor extraction systems showing the constant cross-section influx well geometry. Donnelly (U.S. Pat. No. 4,982,788) shows substantially parallel induction and extraction wells, with immobilization by a condenser and demister. Johnson et al. (U.S. Pat. No. 5,076,727) also disclose a constant flow area injection well, with moist air injection, and microwave/radio frequency heating of the soil to volatilize nonvolatile hydrocarbon contaminants. Johnson et al. (U.S. Pat. No. 5,114,497) show the same injection well geometry as in their patent cited immediately above, except that the soil is thermally heated and a vapor separation process is included. Graves et al. (U.S. Pat. No. 5,178,491) show constant-area cylindrical wells, here the flow consisting of a vapor phase nutrient for bio-remediation. Bentley (U.S. Pat. No. 5,332,333) discloses a cylindrical downflow conduit perforate at its lower end, with means to prevent contaminated water from being brought to the surface.
The vapor extraction process uses the stripping of volatile organic contaminants from soils by an air stream through the contaminated soil zone. The air velocity through the pore spaces of the contaminated zone gives rise to shear action which strips volatile compounds from the soil and entrains them into the air stream. When the air velocity is increased, the shear action within the soil increases and so does the rate at which the volatile compounds are stripped from the soil.
The state-of-the-art technology as shown in the above-mentioned patents relies on constant flow area air influx wells. With such a geometry and a given size of air displacement apparatus there is a limit to the air velocity within the air influx wells and through the contaminated soil zone, and hence to the rate of stripping of volatile compounds. It is an object of the present invention to produce higher air velocities and higher rates of stripping of volatiles for a given size air displacement apparatus by changes in the geometry of the flow system. Specific objects appear below.